The heart is the center of a person's circulatory system. It includes a complex electro-mechanical system that draws oxygenated blood from the lungs and pumps it to the organs of the body to provide the organs with their metabolic needs for oxygen, and draws deoxygenated blood from the organs and pumps it into the lungs where the blood gets oxygenated. In a heart having a normal electrical system, the sinoatrial node, the heart's natural pacemaker, generates electrical signals, called action potentials, that propagate through an electrical conduction system to various regions of the heart to excite myocardial tissues in these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various regions of the heart to contract in synchrony such that the pumping functions are performed efficiently. The function of the electrical system is indicated by a biopotential signal sensed with at least two electrodes attached on the skin or implanted in the body. When the electrical system functions abnormally, the biopotential signal shows that contractions at one or more cardiac regions are chaotic and asynchronized. Such conditions are known as cardiac arrhythmias. Timing and morphological information contained in the biopotential signal is used to diagnose the type of arrhythmia and/or determine an appropriate therapy.
When the biopotential signal is sensed with electrodes attached onto the skin, the sensed signal is commonly referred to as surface electrocardiogram (ECG), or simply ECG. Various standard ECG signals (vectors) are recorded for diagnostic purposes with different combinations of electrode locations. When the electrodes are implanted underneath the skin, the sensed signal is referred to as subcutaneous ECG or far-field electrogram. When at least one electrode is placed in or on the heart, the sensed signal is referred to as electrogram or intracardiac electrogram. Surface ECG is widely used for diagnostic purposes and provides for information on the global electrical performance of the heart. Subcutaneous ECG is known to closely approximate the surface ECG. In contrast, intracardiac electrogram indicates localized electrical performance and may not contain sufficient information for general diagnostic purposes. Implantable medical devices such as cardiac pacemakers and cardioverter/defibrillators sense intracardiac electrograms for timing the delivery of therapeutic electrical energy. Though such an implantable medical device is capable of acquiring intracardiac electrograms and transmitting the electrograms for display in an external device, physicians may still need the surface ECG for diagnostic and therapeutic purposes. The skin contact electrodes and the cables connecting the electrodes to an ECG recorder, as required for recording the surface ECG, may become cumbersome, for example, during an operation such as the implantation of the implantable medical device or during a patient examination where ECG is recorded during exercise. Regular in-home ECG monitoring may be impractical in the absence of a physician or other trained caregiver.
While studies have shown that signals acquired with implanted electrodes of certain configurations approximate surface ECG signals, there is a need to implement a system to acquire a signal substituting for various standard surface ECG signals using an implantable medical device.